A power converter is a power processing circuit that converts an input voltage waveform into a specified output voltage waveform. A switched-mode power converter is a frequently employed power converter that converts an input voltage waveform into a specified output voltage waveform. A boost power converter is one example of a switched-mode converter that converts the input voltage to an output voltage that is greater than the input voltage. Typically, the boost power converter is employed in off-line applications wherein power factor correction is required and a stable regulated voltage is desired at the output of the power converter.
A non-isolated boost power converter generally includes an energy storage device (e.g., an inductor) coupled between the input voltage and switching device. The switching device is then coupled to an output rectifier (e.g., a power diode) and an output capacitor. The load is connected in parallel to the capacitor. Again, the output voltage (measured at the load) of the boost power converter is always greater than the input voltage. When the switching device is conducting, the diode is reverse biased thereby isolating the output stage. During this period, the input voltage supplies energy to the inductor. When the switching device is not conducting, the output stage receives the energy stored in the inductor for delivery to the load coupled to the output of the converter.
Analogous to all types of power converters, a boost converter is subject to inefficiencies that impair the overall performance of the power converter. More specifically, the rectifying diode suffers from a reverse recovery condition thereby producing excessive power losses in both the rectifying diode and the switching device and oscillations in both current and voltage therefrom. The effect of the reverse recovery condition is more severe in non-isolated converters, such as the boost power converter. The reverse recovery condition can also detrimentally affect the longevity of the components, especially the rectifying diode and switching device, of the boost power converter. Therefore, efforts to minimize the losses associated with the rectifier and switching device and, more specifically, with the rectifying diode will improve the overall performance of the power converter.
A traditional manner to reduce the losses associated with the rectifying diodes is to introduce a snubber circuit coupled to the rectifying diodes. Snubber circuits are generally employed for various functions including to minimize overcurrents and overvoltages across a device during conduction and non-conduction periods and to shape the device switching waveforms such that the voltage and current associated with the device are not concurrently high. For instance, with respect to rectifying diodes, a snubber circuit may be employed to minimize oscillations in both voltage and current and power losses associated therewith due to reverse recovery currents resulting from a snap-off of the rectifying diode during a transition from a conduction to non-conduction mode of operation.
Snubber circuits are well known in the art. One approach to reduce the reverse recovery currents of the rectifying diode is to employ a snubber circuit that includes an inductor in series with the rectifying diode. This type of snubber circuit attempts to recover the energy stored in the snubber inductor during the reverse recovery period of the rectifying diode for delivery to the output of the converter. While the inductor snubber provides an alternative for reducing the reverse recovery currents of the rectifying diode, there are tradeoffs in the selection of the inductor and auxiliary components of the snubber that detract from the advantages of employing such a snubber circuit.
For medium and high power applications, it may be advantageous to employ a pulse width modulated (PWM) boost converter operating in a continuous conduction mode (CCM). It has been identified, however, that the efficiency and maximum switching frequency of a conventional PWM boost topology is limited by the losses resulting from the reverse recovery currents of the output rectifier. A simple way of minimizing these losses is to limit the rate of change of the current (di/dt) through the output rectifier as it turns off. Different alternatives have been suggested to implement this solution. For cost sensitive applications, a passive snubber (such as the inductor snubber described above) may be used. For applications where higher efficiency is necessary, an active snubber circuit may be employed in the PWM boost converter.
Active snubber circuits generally employ a switch and driving circuitry and energy recovery components. While active snubbers enjoy efficiency improvements over passive snubbers, the efficiencies may still further be improved. This is especially important in view of the more arduous operating conditions for the power converters thereby amplifying the necessity for higher efficiency converters. In many applications, for instance, power supplies are subject to environments having a wide range of temperatures in a convection or conduction cooling environment (i.e., no fans). Therefore, improved converter efficiencies are becoming mandatory to meet difficult conditions thereby further supporting the need for more efficient snubber circuits that temper losses in the converter such as reverse recovery currents from the output rectifier.
Active snubber circuits typically require more elaborate ways of controlling the active snubber switches, which also adds to the complexity of their use over the more operationally simple passive snubber circuits. This is reflected in the necessary driver circuitry used to control the active snubber switches. If one or more of the active snubber switches used is not referenced directly to the "low side" or "current-return side" of the converter, the level of driver complexity increases since that particular active snubber switch drive signal must be referenced to match the requirements of the active snubber switch that it is driving in order to function properly. This operational complexity typically negates some of the advantages gained through the use of active snubber switches and results in increased product costs.
Accordingly, what is needed in the art is a circuit that moderates a reverse recovery current of a rectifier that maintains the advantages associated with lossless snubber circuits, but overcomes the deficiencies presently available in the design thereof, and a cost effective way of driving one or more of the switches therein.